A surface acoustic wave device, which uses a surface acoustic wave being transmitted while energy is concentrated on the surface of a solid, is small, easy to produce and stable in temperature characteristics. Hence, the surface acoustic wave device is used as filters for TV receivers and the like. The surface acoustic wave device generally comprises interdigital transducers formed on the surface of a piezoelectric body. A typical surface acoustic wave device has a pair of interdigital transducers on the surface of the piezoelectric body to generate a surface acoustic wave. An alternating current applied to the input interdigital transducer is converted into mechanical energy on the surface of the piezoelectric body. However, since the electrode is comb-shaped, compression occurs in the piezoelectric body, and an acoustic wave generates. The acoustic wave is propagated through the surface of the piezoelectric body and reaches the output interdigital transducer. The surface acoustic wave having reached the output interdigital transducer is converted further into electrical energy by the output interdigital transducer and then output.
In recent years, the amount of information transmission has increased, and the range of transmission signals is expanding to the microwave range. The demand for devices capable of being used in the range of GHz is increasing. Generally, the operating frequency of a surface acoustic wave device depends on the propagation velocity and wavelength of a surface acoustic wave, and the wavelength is determined by the periodic length of the interdigital transducer. When electrodes having the same periodic length are used, in other words, when surface acoustic wave devices are used at the same wavelength, a surface acoustic wave device having a higher wave propagation velocity inside the material thereof can be used up to higher frequencies. Hence, a method has been proposed wherein a diamond having the highest velocity of sound among all substances (the velocity of the transverse wave: 13000 m/s, the velocity of the longitudinal wave: 16000 m/s) is used as a substrate (for example, Japanese Unexamined Patent Publication No. 64-62911).
Generally, a surface acoustic wave device having a larger electromechanical coupling coefficient (an index of conversion efficiency when electric energy is converted into mechanical energy) operates at higher efficiency. In particular, it is preferable that the electromechanical coupling coefficient is not less than 0.5%. Furthermore, when using the device at high operating frequencies, the propagation velocity of the wave is required to be high.
In a surface acoustic wave device having a structure wherein a ZnO layer is stacked as a piezoelectric body on a diamond layer, it is possible to obtain a device that generates a surface acoustic wave having a high propagation velocity and that has a large electromechanical coupling coefficient by limiting the thickness of the ZnO layer in a specific range (for example, Japanese Unexamined Patent Publication No. 9-51248, Japanese Unexamined Patent Publication No. 10-276061, etc.).
In the case where a piezoelectric thin film formed on a substrate is used, the propagation velocity and electromechanical coupling coefficient depend not only on the material of the piezoelectric thin film and the substrate but also on the thickness of the piezoelectric thin film. Furthermore, when the thin film is used, the quality of the film affects the operation characteristics of the surface acoustic wave device and is required to be excellent in order to attain a high propagation velocity, a high electromechanical coupling coefficient and a low propagation loss. However, in the surface acoustic wave devices disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 9-51248, Japanese Unexamined Patent Publication No. 10-276061, etc., since the thickness of the ZnO layer is relatively small, the quality of the film deteriorates, whereby the propagation loss increases and the electromechanical coupling coefficient becomes smaller than its theoretical value. As a result, proper surface acoustic wave devices are not obtained.
In view of the above-mentioned situations, the present invention is intended to provide a surface acoustic wave device comprising a diamond and having a structure wherein a ZnO layer is formed on the diamond, having operating frequencies in the range of several hundreds of MHz to several tens of GHz, and being capable of selectively using high operating frequencies.